1. Field of the Invention
The present invention relates to a mechanism for adjusting the distance between a light source and a collimating lens in the optical axis direction in a light source apparatus of an optical scanning apparatus that can be used for image recording apparatuses such as laser printers, multifunction devices and facsimile devices, and to a method for producing the mechanism.
2. Description of Related Art
FIGS. 24A and 24B are diagrams schematically showing a configuration of an example of a related art optical scanning apparatus in a laser beam printer. FIG. 24A is a perspective view and FIG. 24B is a top view showing a configuration of the main optical components.
In the following, an outline of the operations of the optical scanning apparatus is briefly described with reference to FIG. 24B. The divergent light emitted from a light source (semiconductor laser) 101 passes through a collimating lens 102, a diaphragm 103 and a cylindrical lens 104, reflected and scanned at a polygon mirror 105, passes through an fθ lens 106, and is imaged and scanned at constant speed on a photosensitive material 107. In a light source apparatus 110, the divergent light emitted from the light source is converted into parallel light beams at the collimating lens 102, and the light beams are shaped by passing through the diaphragm 103.
Here, the conditions required for the light source apparatus 110 constituted by a light source 101, the collimating lens 102 and the diaphragm 103 are that the optical axis of the light source 101 and the optical axis of the collimating lens 102 coincide with the desired optical axis of the light source apparatus 110, and that the distance between the light source 101 and the collimating lens 102 is adjusted such that laser light 109 that is output from the light source 101 is converted into parallel light beams by the collimating lens 102.
In general, in an optical scanning apparatus of an under-field optical system, a collimating lens 102 having a focal length of about 6 mm to about 15 mm is selected. When the focal length of the collimating lens 102 is increased, the width of the parallel light beams after passing through the collimating lens 102 increases. When the diaphragm 103 is made larger correspondingly, a large polygon mirror 105 will be required. In that case, problems relating to electric power, starting time, noise, heat and the like will arise.
On the other hand, when the diaphragm 103 is not made large, the amount of light passing through the diaphragm 103 decreases, so that a light source 101 with a large output will be required. In addition, there will be a large amount of vignetting of the light beams at the diaphragm 103, and therefore the beam diameter of the optical scanning apparatus increases due to the influence of the beam diffraction.
However, when the optical scanning apparatus is placed in a laser printer, a charging apparatus, a developing apparatus, a transfer apparatus, a cleaning apparatus for the photosensitive material and the like are present around the photosensitive material 107, and therefore the optical scanning apparatus is placed some distance from the photosensitive material 107. Accordingly, an fθ lens 6 having a focal length of about 100 mm to about 200 mm naturally will be used for the optical scanning apparatus.
Here, when the focal length of the collimating lens 102 is set to 10 mm, and the focal length of the fθ lens 106 is set to 200 mm, the lateral magnification is 20 times and the longitudinal magnification is 400 times, since the lateral magnification is equivalent to the focal length ratio and the longitudinal magnification is equivalent to the square of the focal length ratio. This means that when there is an error of 10 μm in the optical axis alignment, the positional displacement on the photosensitive material 107 is 200 μm, and when there is similarly an error of 10 μm in the adjustment of the distance between the collimating lens 102 and the light source 101, the focus displacement on the photosensitive material 107 is 4 mm.
That is to say, when a collimating lens having a short focal length is selected as the collimating lens 102, the magnification is even higher, resulting in an even more serious error in the adjustment of the light source apparatus 110. Since the magnification of the light source apparatus 110 of the optical scanning apparatus is high in this way, the precision in the adjustment and the stability after the adjustment are important and an extremely high reliability is required particularly in the adjustment of the distance between the collimating lens 102 and the light source 101.
Under such circumstances, various methods for adjusting the distance between the collimating lens 102 and the light source 101 have been proposed for ensuring the adjustment precision and decreasing the number of man-hours for adjustment. These methods broadly can be divided into a method of making an adjustment by moving the collimating lens 102 parallel to the optical axis direction while keeping the light source 101 fixed, and conversely a method of moving the light source 101 parallel to the optical axis direction while keeping the collimating lens 102 fixed.
FIG. 25 shows a typical example of a distance adjustment method in which a semiconductor laser is fixed and a collimating lens is movable (see JP 63-162310 U (page 1, FIG. 1)). A collimating lens that has been inserted and fixed in a lens barrel 111 in advance is placed on a V-groove 113 of a holding member 112 and moved back and forth in the optical axis direction to adjust the distance to the light source 114. In addition, with this configuration, the adjustment of the optical axis is performed by finely adjusting the position at which a light source fixing member 115 is bonded to the holding member 112.
However, with this method, the optical axis may be displaced when performing the distance adjustment by moving the collimating lens back and forth in the optical axis direction, or the distance to the light source 114 may be shifted when, conversely, moving the light source fixing member 115 on the plane perpendicular to the holding member 112 and the V-groove 113 for adjustment, if precision has not been achieved, for example, for the parallelism between the two planes forming the V-groove 113, their respective flatness, the flatness of the perpendicular plane of the holding member 112 to which the light source fixing member 115 is mounted, and the perpendicularity between the perpendicular plane of the holding member 112 and the V-groove 113.
Moreover, the optical axis may be displaced when performing the distance adjustment, if a sufficient precision has not been achieved also for the fixation of the lens to the lens barrel 111. If the lens barrel 111 were to be omitted in order to reduce the cost, the collimating lens would become unstable on the V-groove 113 due to its small thickness, making it difficult to achieve precision or to reduce the number of man-hours for adjustment.
That is, this configuration requires a high precision for the holding member 112, the lens barrel 111 and all the components for fixing the lens to the lens barrel 111, and takes a large number of man-hours if the precision cannot be ensured. This structure is simple and therefore is of low cost in terms of the materials, but is of high cost in terms of the quality control for the components, the yield and the number of man-hours.
Furthermore, when the pressing force for bringing the lens barrel 111 and the V-groove 113 into close contact is small, a gap is formed between the edge of the V-groove 113 and the lens barrel 111 when moving the lens barrel 111 back and forth, which may cause a displacement of the lens optical axis. However, when the pressing force is large, the friction between the lens barrel 111 and the holding member 112 increases, thus making it difficult to smoothly move the lens barrel 111 back and forth during the distance adjustment and increasing the adjustment operation time.
When an inexpensive molded resin is used for the holding member 112, the pressing force of an elastic member 116 may cause the holding member 112 to undergo creep deformation under a high temperature environment, which places limitations on the selection of materials. Furthermore, there are also the problems of a displacement due to expansion or constriction of the members caused by temperature changes and a displacement due to vibrations, impact forces and the like from the outside.
In view of these considerations, a measure that is generally taken is to inject an adhesive into the space between the lens barrel 111 and the holding member 112 to prevent displacement. However, this does not provide a fundamental solution. The reason is that the adhesive undergoes a volume change when cured and therefore the distance to the optical axis or the light source 114 may be shifted when the adhesive is cured after completion of the adjustment.
A distance adjustment method in which, conversely to the above-described configuration, the lens is fixed and the light source is movable also has been proposed (for example, see JP 2001-264669 A (pages 3 to 4, FIG.3)). According to the method of JP 2001-264669 A, a light source (semiconductor laser) 401 is fixed to a casing 402, which is a hollow screw, and the whole structure is fixed to a base 405 with two nuts 403 and 404, as shown in FIG. 26.
However, this method in which the position of the light source is fixed by tightening the two nuts 403 and 404 has several drawbacks as the adjustment operation. First, the light source rotates at the time of performing the distance adjustment. Since the longitudinal divergence angle and the lateral divergence angle of the laser light from the light source 401 are different, rotation of the light source causes the beam diameter to deviate from an optimum value due to the influence of an eclipse in light beams at the diaphragm. Accordingly, it is necessary to provide a separate device that is not described in this document to prevent the light source from rotating.
Further, the tightening torque load of the nuts 403 and 404 causes deformation of various components and variations between the components, such as play and backlash. The focus position changes subtly depending on the magnitude of the tightening torque. When the tightening torque of the nuts 403 and 404 is small, loosening of the screw occurs owing to impacts during transportation and vibrations inside the printer apparatus, thermal stress by temperature variations, so that the quality and the reliability of the product will be reduced significantly unless the management of the torque is performed.
When the structure is assembled with an excessive tightening torque, creep deformation due to heat leads to degradation of the quality over time. Furthermore, the fact that the tightening torque causes fluctuations of the focus position also leads to an increase in the number of man-hours for adjustment. The adjustment procedure for the case where the distance between the lens and the light source 401 is too close is discussed below. The nuts 403 and 404 behind the light source 401 are loosened temporarily, thereafter the front nut 403 is rotated by a desired angle to displace the light source 401 backward, and the rear nut 404 is tightened again. However, in this method, the position of the light source 401 and the relative positions of the nuts 403 and 404 and the screw are fixed by the fastening power of the two nuts 403 and 404, so that the front nut 403 actually is loosened slightly upon loosening the rear nut 404.
Accordingly, when displacing the light source 401 backward, it is necessary to rotate the front nut 403 by an extra amount to move the light source 401 backward by an extra amount, taking in consideration that the front nut is loosened. The degree of this operation is influenced, for example, by variations of components and the skill level of the operator, resulting in variations in quality, a decrease in reliability and an increase in the number of man-hours.
Another method in which the lens is fixed and the light source is movable as in the above-described method also has been proposed (Japanese Patent No. 3077375 (pages 4 to 5, FIG. 1 and FIG. 3)). FIGS. 27A to 27C show this adjustment method and the adjustment mechanism. FIG. 27A is an exploded perspective view of a light source apparatus. FIG. 27B is a rear view thereof, and FIG. 27C is a cross-sectional view taken along the line G-G in FIG. 27B.
The semiconductor laser 501 is positioned by a semiconductor laser positioning member 502, and an adjusting screw 504 is mounted to the central position of an adjusting screw holding member 503. A spacing member 508 retains an interval between a terminal 506 of the semiconductor laser 501 that is electrically connected to a signal cable (flexible cable) 505 and the adjusting screw 504, thus maintaining an electrically insulated state.
Moving the adjusting screw 504 back and forth causes a semiconductor laser holding portion 507 to which the semiconductor laser 501 mounted to move back and forth in the optical axis direction via the spacing member 508, thus adjusting the interval between the semiconductor laser 501 and a collimating lens 509.
It should be noted that the adjustment of the optical axis is carried out within the range of the outer diameter of fixing screws of 510 and 511 of the semiconductor laser unit and the gap between mounting holes 512 and 513, into which the fixing screws 510 and 511 are inserted.
Accordingly, the position of the semiconductor laser 501 may be adjusted, for example, by setting a jig for detecting a laser beam at the target position of the adjustment and moving a semiconductor laser mounting member 514 either in the main scanning direction or in the sub scanning direction, and the fixing screws 510 and 511 of the semiconductor laser unit may be tightened in a state in which the laser beam is in focus at a desired position.
However, this method has several problems as shown below. With this adjustment structure, the spacing member 508 is disposed on the back surface (the terminal side) of the semiconductor laser 501, so that it is structurally impossible to directly connect the circuit board to the semiconductor laser terminal. Since the circuit board cannot be mounted directly to the semiconductor laser 501, an electrical contact point is secured via the signal cable 505.
With this configuration, the electrical resistance and the capacitance of the signal line are increased, and therefore the response speed of the semiconductor laser 501 that is driven at high speed is reduced. Furthermore, the signal cable 505 is unstable and difficult to handle. For this reason, component failures such as a solder detachment and electrostatic breakdown of the semiconductor laser 501 may be induced during the transportation process of half-finished products or the assembling process of the light source apparatus.
Additionally, with this configuration, the adjusting screw 504 is in point contact with the spacing member 508, which maintains the adjustment position, so that the inclination of the elastic member tends to be unstable. When the position at which the adjusting screw 504 is in contact with the spacing member 508 is displaced from the center of the spacing member 508, a rotation moment may be generated on the spacing member 508, thus causing the elastic member to incline. This means that when the distance adjustment is performed after adjusting the optical axis, the optical axis will be displaced again. This problem occurs when the tip portion of the adjusting screw 504 is off-center or when the precision of the processing of the spacing member 508 and the assembly of the semiconductor laser unit is poor, leading to an increase in the number of man-hours in such a case. As a result, it is necessary to manage the components and the assembly precision, which causes an increase in the manufacturing cost, including the number of man-hours.
In addition, the fact that the contact between the adjusting screw 504 and the spacing member 508 is a point contact means that they are very unstable against vibrations caused by external forces. Vibrations, impact forces and the like that are applied from the outside during the laser scanning, and vibrations inside an apparatus that rotates at a high speed, such as a polygon motor, cause the semiconductor laser 501 to vibrate, thus possibly causing jitter and color drift in a print image.
In this configuration, a force countering the deformation of the elastic member is exerted on the semiconductor laser 501 and the adjusting screw 504, and this provides the effect of absorbing backlash of the adjusting screw 504. However, the adjustment of rotation of the adjusting screw 504 is carried out from behind the adjusting screw holding member 503, and therefore pressing a tool against adjusting screw 504 from behind for rotating the adjusting screw 504 causes the semiconductor laser 501 to be moved by an amount corresponding to the backlash of the adjusting screw 504. Also in this method, the operator is required to have a considerable level of skill. When the level of skill of the operator is low, the adjustment period will be long and the adjustment precision will be poor.
Further, when the temperature inside the apparatus fluctuates, the collimating lens base expands and contracts to cause fluctuations in the distance between the collimating lens 509 and the semiconductor laser 501, which may displace the focus position. This problem occurs especially when an inexpensive resin molded product is used in place of the collimating lens base.
Although it is possible to decrease the linear expansion coefficient by selecting a resin material containing reinforced fibers such as glass fibers, the orientation may change depending, for example, on the molding conditions of the resin, so that the linear expansion coefficient may not be of a desired value. In that case, with this configuration, it is necessary to correspondingly change the linear expansion coefficient by using varied raw materials for the collimating lens base itself. However, due to the complex shape of the collimating lens base, it is difficult to achieve a desired value of linear expansion coefficient by changing the molding conditions or the material.
Furthermore, this configuration requires a large number of components, including for example, the spacing member 508, the semiconductor laser mounting member 514, the adjusting screw holding member 503 and the signal cable 505, and thus is an expensive structure in terms of both the material costs and the number of man-hours for assembly. Additionally, it should be appreciated that the size of the light source apparatus will increase since the adjusting screw 504, the adjusting screw holding member 503 and the like are disposed behind the semiconductor laser 501.